Acoustic Underlay Selection Guide

Quick Answer: Acoustic underlay is specified by its impact sound improvement value (ΔLw), measured in decibels — higher is better. For a new flat conversion or dwelling above another, Building Regulations Part E requires the combined floor assembly to achieve ≤65 dB impact sound level (Ln,w,eq). A typical 6–8mm PU foam or crumb rubber underlay achieves ΔLw of 17–25 dB. The floor construction, not just the underlay, determines compliance — always check the acoustic test data for the complete system.

Summary

Impact sound — footsteps, dropped objects, chair scraping — is the dominant noise complaint in UK flatted dwellings. The legal requirement for new flats (converted or purpose-built) under Building Regulations Part E is clear: the floor assembly must meet a minimum standard of 65 dB Ln,w,eq (weighted standardised impact sound pressure level). But many installers select acoustic underlay based on feel or price rather than published performance data, and then find the floor assembly fails testing.

The fundamental principle is that acoustic underlay does not act alone. Its performance depends on the interaction between the structural floor (concrete slab or timber joist), the ceiling below (if tested in-situ), the floor finish above, and the underlay. A high-specification underlay under a ceramic tile will perform very differently from the same underlay under a floating laminate, because tile adhered to screed does not allow the underlay to function as a resilient layer. For hard floor finishes in flatted properties, decoupling systems and acoustic mat products are required rather than conventional underlay.

This guide focuses on acoustic underlay for carpet, laminate, and floating engineered wood in UK residential applications.

Key Facts

• Building Regulations Part E — applies to new dwellings and material change of use (e.g. converting a house into flats); target Ln,w,eq ≤65 dB for impact sound
• ΔLw — weighted impact sound improvement value; the performance metric for underlay. Measured per BS EN ISO 717-2
• Ln,w — weighted impact sound transmission level of the complete floor construction; what is actually tested in-situ
• Material change of use — converting an existing house into flats requires Part E compliance; the existing floor construction must be upgraded to meet the target
• Part E does not apply to like-for-like replacements — replacing carpet with carpet in an existing flat does not trigger Part E compliance
• Floating floor is required — glued-down or nailed-down finishes (hardwood, tile) do not benefit from resilient underlay; the finish must float for the underlay to reduce impact transmission
• PU foam underlay — most common; density 100–140 kg/m³; ΔLw typically 17–23 dB; available in 3–12mm depth
• Crumb rubber underlay — denser and firmer; ΔLw typically 22–28 dB; suited to heavy traffic and commercial installations
• Felt underlay — traditional material; natural or synthetic; stiffer than foam; ΔLw 15–22 dB; good for Axminster and Wilton carpet
• Combination felt/rubber — layers of felt and rubber; ΔLw 22–26 dB; performs well under carpet in luxury installations
• Timber stud (suspended) floors — impact attenuation is inherently lower than solid concrete; compliance typically requires specialist resilient bar ceiling system plus acoustic quilt plus dense screed/platform floor above
• Minimum underlay thickness for carpet — 8mm for domestic bedroom; 10mm for living areas; 6–7mm for stairs
• Tog rating — the thermal resistance of underlay; relevant for rooms over electric UFH mats where the underlay adds unwanted insulation

Quick Reference Table

Detailed Guidance

Regulatory Compliance in Flats — What You Must Check

Building Regulations Part E requires pre-completion testing in new dwellings and converted flats under Approved Document E. For a converted flat, the owner or builder must demonstrate compliance either through:

• Robust Details — following a pre-approved construction detail from the Robust Details scheme (listed at robustdetails.com); no in-situ testing required if the detail is followed exactly
• In-situ testing — acoustic testing by an accredited body (e.g. UKAS-accredited laboratory) that measures the completed floor

For a floor that incorporates carpet and underlay, the relevant Robust Detail is typically E-FC-3 or E-FC-4 (floating carpet over concrete). For laminate or engineered wood, E-FC-5. Each Robust Detail specifies minimum underlay performance, floor construction, and ceiling treatment — you cannot mix and match components from different details.

If a Robust Detail is not followed precisely, in-situ testing is required. Failing the test means the floor must be stripped and remediated — an expensive mistake.

Selecting Underlay for Carpet in Flats

For carpet in a flatted property subject to Part E:

1. Identify whether a Robust Detail applies (check robustdetails.com for the specific floor/ceiling type)
2. If using a Robust Detail, check the required underlay specification — typically crumb rubber ≥6mm at ≥140 kg/m³ or equivalent ΔLw
3. If not using a Robust Detail, commission acoustic testing after installation to verify compliance before completion sign-off
4. The carpet itself also contributes to acoustic performance — denser carpet with higher pile weight performs better

Underlay for Floating Laminate and Engineered Wood

Many laminate and engineered wood products have pre-attached underlay — a thin foam or cork backing. This is not equivalent to a separate acoustic underlay and typically achieves only ΔLw 10–15 dB. For flats, always use a separate acoustic underlay below the flooring panel, not in addition to the integral backing (double underlay creates instability and can void the floor warranty — check manufacturer guidance on integral + separate underlay compatibility).

For floating engineered wood:

• Underlay thickness 3–5mm typical (thicker underlay under floating planks causes instability at click joints)
• Density ≥120 kg/m³ recommended
• Vapour barrier built in or laid separately (required on concrete substrates)

Underlay for Underfloor Heating

Electric UFH mats embedded in adhesive (ceramic tile systems) do not benefit from conventional underlay — the tile is bonded directly to the mat.

For wet UFH under floating engineered wood or laminate:

• Maximum underlay tog value: 0.15 tog recommended; some manufacturers state maximum 0.25 tog
• High-tog underlays insulate against the UFH output and significantly reduce efficiency
• Use a low-tog acoustic underlay (typically crumb rubber or dense foam 3–5mm) — these are denser, thinner, and therefore lower in thermal resistance than conventional foam

For electric UFH under carpet:

• Always check the combined carpet + underlay tog rating against the UFH manufacturer's limit
• Typical maximum combined carpet + underlay: 1.5 tog; check the UFH manufacturer's guidance
• High-tog underlay + thick carpet over electric UFH can cause the heating element to overheat and fail

Underlay for Stairs

Acoustic underlay for stairs is not about Part E compliance (stairs are typically excluded from Part E calculations). It is about durability and feel:

• Underlay on stairs must be ≥6mm and ≥100 kg/m³ — thin or soft underlay compresses at the nosing and accelerates carpet wear at that point
• Crumb rubber is preferred for stairs over foam — more resilient to the point loading at the nosing
• Felt underlay is also commonly used on stairs, particularly under Axminster and Wilton runner carpets

Frequently Asked Questions

Does acoustic underlay under laminate in a flat guarantee Part E compliance?

No. A single underlay product cannot guarantee compliance — the complete floor assembly (slab, screed, ceiling treatment, underlay, floor finish) must meet the target as a system. Even a high-specification underlay may not achieve compliance if the ceiling below is not treated. Always check the applicable Robust Detail or arrange acoustic testing.

Is crumb rubber better than foam for acoustic performance?

Crumb rubber typically achieves 2–5 dB better ΔLw than foam of equivalent thickness due to its higher density and resilience. The difference is audible but not dramatic for a single occupant. However, in a flat where noise complaints are at risk, the additional 3 dB from crumb rubber versus foam can be the difference between passing and failing an acoustic test. For compliance applications, specify crumb rubber or combination underlay.

Can I use acoustic underlay under hardwood flooring that is glued or nailed down?

No. Acoustic underlay only works when the floor finish is floating — free to act as a resilient decoupled layer. Glued or nailed-down floors transmit impact directly to the structure. For glued hardwood in a flat, a specialist acoustic mat (rebonded foam bonded to a non-woven carrier) is sometimes used under the wood, but it must be approved by the floor manufacturer and is not compatible with all adhesive systems.

What thickness of underlay do I need under carpet?

For domestic use: 8–10mm PU foam or equivalent is suitable for bedrooms and living areas. For stairs: 6–7mm crumb rubber or dense felt. For heavy commercial traffic: 10–12mm crumb rubber. For acoustic compliance in a flat: check the Robust Detail specification, which typically specifies density rather than thickness.

Regulations & Standards

• Building Regulations Part E (England/Wales): Resistance to the passage of sound — Approved Document E sets out the Ln,w targets for impact sound in new and converted dwellings

• BS EN ISO 717-2 — Acoustics; Rating of sound insulation in buildings; Part 2: Impact sound insulation (the standard under which ΔLw is measured)

• Robust Details Ltd scheme — pre-approved construction details for Part E compliance without in-situ testing (visit robustdetails.com for the current list)

• Building Regulations Part L — Thermal performance; relevant for underlay tog selection over UFH

• Approved Document E — gov.uk — full text of Approved Document E

• Robust Details Ltd — scheme details and registered construction types

• Contract Flooring Association: Acoustic Guidance — acoustic underlay selection and testing guidance for UK floor installers

• carpet fitting: gripper rods, stretching, and seaming techniques — fitting carpet over acoustic underlay

• LVT installation guide — acoustic considerations for hard floors in flats

• tile backer boards including acoustic decoupling mats — acoustic performance for tile finishes